As always, everyone will answer your question differently. I happen to agree with the two posters above - Berlin can fill as many days as you can spare. Like with London, or Paris, or New York, you'll never run out of things to do there (at least I wouldn't). Of course, if you don't like big cities, or if you're not particularly drawn to twentieth century history, or you like your cities "pretty" and "picturesque," then you'll want to focus time elsewhere.
A personal story: years ago, before I had really traveled anywhere, a coworker told me "You must go to Jerusalem - it's history under your feet." When I went to Berlin, I thought of these words CONSTANTLY. I was aware of the walking on the 20th century in a very visceral way, as well as seeing reminders of the many changes the city underwent in that time (monarchy, democracy, Nazis, communists, division, reunification, gentrification, etc) and the many groups woven into the city's story (Jews, gays, Turks, etc). Not everyone responds to the same things, but for me, Berlin is amazing. (Jerusalem, when I finally got there, ironically made much less of an impression on me than Berlin did).